In the top blown basic oxygen steel making process, a vessel is charged with a liquid carbon saturated iron alloy referred to as hot metal, scrap steel, and fluxes that provide CaO and MgO to the process. A water-cooled lance is inserted into the vessel through which oxygen is injected at supersonic speeds. The lance has at least one port and often multiple ports at the tip through which the oxygen exits and impinges onto the surface of the charge. The oxygen reacts with the metallic and carbon components of the charge, and heat is generated by the exothermic reactions. Over time, the oxygen reacts chemically and oxidizes substantially all of the silicon and aluminum that were present in metallic form in the charge.
In addition, most of the carbon in the charge is oxidized and the typical finished raw steel has a carbon content of between about 0.02% and about 0.06%, at which concentration the liquid steel is referred to as a flat bath. As the carbon approaches this low level, the oxygen also reacts with manganese and iron in the charge. At the flat bath condition, much of the manganese is oxidized and is present as MnO in the slag. Also at flat bath, the iron is oxidized to an extent that approaches equilibrium with the oxygen concentration in the steel. For example, oxygen content in the steel may reach about 0.08% with iron oxide concentration at about 28% in the slag at the conclusion of the blowing process. The slag is formed by the dissolution of the oxide components within each other, and may have about 40% CaO, 26% FeO, 10% SiO2, 10% MgO, 5% Al2O3, 5% MnO and some other minor components making up the balance.
This slag can act beneficially to remove phosphorus and other impurities from the steel. The process of oxidation, heat generation and refining is complex and is monitored and controlled typically by a process model. The process model attempts to take into account the mass balance, thermal balance, thermodynamic reactions and kinetic rates to predict the end point and achieve the desired result in the shortest time and with the least cost. Many factors that cannot be accurately measured have influence on the process and therefore the process model is usually inadequate to cause a desired outcome every time. As a result, sometimes a re-blow is required to adjust the chemistry or temperature of the final steel. This is costly and time consuming. In addition, the process may cause slopping of the charge and ejection of steel, which results in yield loss and is costly. Slopping is an oscillation of the charge from side to side within the vessel, such that the charge advances and recedes along opposed portions of the vessel wall. When the slopping becomes extreme, the charge can surge over the upper rim of the vessel, resulting in an ejection of molten steel and slag therefrom.
There are many factors that can influence slopping and ejection of material from the basic oxygen furnace, commonly referred to as the BOF. Among them are the rate of oxygen injection, the silicon content of the charge, the height of the lance above the bath, the volume of the bath in comparison with the volume available in the BOF, the shape and aspect ratio of the BOF interior, the temperature of the bath, the extent to which carbon monoxide (CO) compound is further oxidized to CO2, the wear of the lance tip ports, the shape and stability of the cavity formed by the oxygen impingement force, the extent of emulsification of metallic and oxide phases, and the chemical composition of the slag.
The problem of ejection of material due to slopping within the furnace is well known in the art and there have been many attempts at characterization and mitigation of this problem. It has been observed that slopping begins about 30% to about 60% of the way through the oxygen blowing period after the silicon in the charge is oxidized, and the slag becomes fluid and the CO generation rate is near its peak. In U.S. Pat. No. 5,584,909, Kim teaches reducing the oxygen blowing rate and the lance height above the bath near the peak CO generation period in order to prevent slopping. While this may be effective, it may slow the process and limit production rates. Also, the time at which the actions of reducing the blowing rate and the lance height need to be implemented are variable and not well known.
Another method of mitigation of slopping is to attempt to control the slag chemistry within the BOF. For example, it is thought that excess iron oxide can be formed when the bath penetration by the oxygen jet is not deep enough. The excess iron oxide can influence slag chemistry and may increase the amount of slopping. In U.S. Pat. No. 4,473,397, Bleeck, et al. teach the addition of calcium carbide to the slag within the BOF as slopping begins to reduce excess FeO content, thereby reducing the degree of slopping. The reagent calcium carbide is expensive and the effective amount can be variable. In addition, the optimal time of addition may not be known, so the reagent may be consumed prior to the actual time that it is needed. For these and other reasons, this method is not commonly used in the art.
The onset of slopping is typically preceded by a high rate of gas generation into the slag that causes foaming and rising of the slag toward the top of the BOF vessel. Therefore, it is believed that if the level of the slag within the vessel can be monitored, then the onset of slopping can be predicted. To this end, in U.S. Pat. No. 4,210,023, Sakamoto et al. teach the use of a microwave measuring apparatus to determine the height of the foaming slag within the BOF vessel. In practice, the microwave device is difficult to maintain due to the harsh environment within the BOF vessel. In U.S. Pat. No. 5,028,258, Aberl et al. teach the use of sound pick up devices to monitor sound emanating from the BOF vessel. The oxygen blowing onto the charge generates a sound, which is attenuated by the slag as it foams and rises up the length of the lance. Aberl et al. have correlated the amount of attenuation to the level of the slag as it rises within the vessel, so that mitigating action can be taken prior to the onset of slopping. In practice, there are many aspects that may influence the speed, frequency or intensity of sound that reaches the pick up device, including temperature and dust generation levels. As a result, the accuracy and efficacy of this method may not be sufficient. In addition, the pick up devices are prone to failure due to the harsh environment in which they are installed.
One aspect of slopping within the BOF vessel is the vibration of the vessel and the lance due to the momentum of the charge during the slopping event. The momentum may cause significant vibration in both the vessel and the lance assembly. In U.S. Pat. No. 4,398,948, Emoto et al. teach the monitoring of horizontal movement of the BOF lance with an accelerometer. The slopping action within the furnace causes the slag to impact the lance that causes horizontal movement and the extent of this horizontal lance acceleration is correlated to the extent of slopping within the furnace. While this method is simple and effective, some problems are associated with it. The single axis horizontal acceleration is sometimes insufficient to indicate the extent of slopping due to the impact angle and momentum variance on the lance in the furnace. The amount of slopping measured is not related to the amount of material ejected from the furnace or to the loss of iron units. Therefore, it is not determined exactly when to take mitigating measures against slopping. Thus the method is not predictive of slopping, but rather is indicative of slopping events already underway.
While not wishing to be bound by any particular theory, the applicants have determined that there is a frequency of interest in monitoring the lance vibration that is indicative of the impact of the oxygen jet into the impingement cavity. The intensity of this vibration is attenuated as the foaming slag rises up the length of the oxygen lance. By monitoring two frequencies, a higher one that is indicative of the vibration caused by the oxygen impact within the impingement cavity and a lower one that is indicative of the vibration of the lance due to impact by the slopping charge, more useful information is gleaned. (This concept was presented at the 2005 Association for Iron and Steel Technology conference in Charlotte, N.C. in a paper entitled “Vessel Slopping Detection”, coauthored by the present inventors.)
The high frequency range amplitude attenuation was found to precede and be indicative of the impending slopping event evidenced by the low frequency range amplitude increase. This was an important finding since the mitigating action can now be taken prior to the actual onset of slopping and its effectiveness can be measured by monitoring the intensity of slopping at the same time. However, there are still deficiencies in the method as presented in the referenced paper. There is no absolute indication that relates the slopping intensity to the timing and amount of material ejection from the furnace. There is some acceptable level of slopping in all operations, and there is a desire to minimize process time and therefore maximize oxygen blow rate. However, the method of the aforementioned paper does not address what level of slopping is acceptable in the interest of maximizing steel production, while simultaneously minimizing cost. Furthermore, to the best of the applicants' knowledge, there is no quantitative correlation developed between the oxygen blow rate, lance height and slopping in the known art.
There remains a need for an apparatus and method of steelmaking in a basic oxygen furnace that can detect the onset of slopping, and then adjust the process conditions to prevent the slopping from causing ejection of steel from the vessel, while maintaining the desired chemistry of the charge, and throughput of conversion to finished steel ready for a pour. There is a further need for apparatus and method of steelmaking in a basic oxygen furnace that can more reliably detect the end point of the steelmaking process, such that excessive oxygen content is not introduced into the steel.